1. Field of the Invention
The invention relates to an ultraviolet irradiation device which is used for bonding an article to be treated which frequently undergoes changes, such as deformation, heat-induced color changes and the like, using UV radiation, or for curing of ink and the like, the article being a plastic, thermal paper, liquid crystal and the like.
2. Description of Related Art
Conventionally, the following is known as an ultraviolet irradiation device which reduces unnecessary radiation in the visible range or the infrared range and which can carry out curing by suppressing deformations, color changes and the like of the article to be treated:
(1) a combination of a rod-shaped lamp, such as a high pressure mercury lamp, a metal halide lamp or the like, with a cold mirror, (a mirror which has the properties of reflecting ultraviolet light and transmitting or absorbing visible rays and especially infrared light) or
(2) a combination of the above described rod-shaped lamp with the cold mirror and a heat refraction filter of the air cooling type or water cooling type (a filter which has the properties of transmitting ultraviolet light, reflecting visible rays and absorbing some of the infrared light).
FIG. 6 is a schematic illustration of the energy of the light which is emitted by the ultraviolet irradiation device described above in (2), in which the cold mirror 2 is combined with a heat refraction filter 3 of the air cooling type. In the representation, a rod-shaped lamp 1 which, for example, has a tube diameter of 18 mm and an input power per unit length of 240 W/cm. Rod-shaped lamp 1 is subjected to air cooling by a means which is not shown in the drawing. Furthermore, reference number 1a indicates the arc area of the rod-shaped lamp 1. FIG. 6 is a transverse cross-sectional representation in which the rod-shaped lamp 1 has been cut perpendicular to its major (longitudinal) axis.
Furthermore, the cold mirror 2 is trough-like and is arranged parallel to the major axis of rod-shaped lamp 1. Cold mirror 2 is made of a glass or the like which is provided with a vacuum evaporation film which reflects ultraviolet (UV) light and which transmits visible rays and infrared light. A plate-shaped heat refraction filter 3 is made of a glass which transmits ultraviolet light, but which does not transmit infrared light of wavelengths greater than or equal to 3 to 4 microns. Still further, heat refraction filter 3 can also be formed from a glass which is provided with a vacuum evaporation film which transmits ultraviolet light and which reflects visible rays. Reference letter W designates a workpiece which is irradiated with radiant light from an ultraviolet irradiation device which consists of the above described parts and to which ink or the like has been applied.
As is shown in the drawing, some of the light which is emitted from arc area 1a of rod-shaped lamp 1 is incident on cold mirror 2, while the remainder is incident on heat refraction filter 3. Furthermore, infrared light is emitted from the surface of rod-shaped lamp 1 because it has a high temperature, and some of this infrared light is incident on cold mirror 2 and the remainder on heat refraction filter 3.
Of the light incident on cold mirror 2, some of the visible rays and infrared light pass through cold mirror 2, and ultraviolet light (including some of the visible rays and infrared light) is reflected by cold mirror 2 and is incident on heat refraction filter 3. Furthermore, of the light which has been emitted from rod-shaped lamp 1 and which is incident directly on heat refraction filter 3, some of the visible rays are reflected by heat refraction filter 3. As is shown in the drawing, workpiece W is, therefore, irradiated with ultraviolet light, some of the visible rays, and some of the infrared light.
In this case, of the light emitted onto the workpiece W, the light which contributes to setting of the ink or the like applied to the workpiece is ultraviolet light. The visible rays and the infrared light increase the workpiece temperature, but do not contribute to setting of the ink applied to the workpiece or the like. Therefore, it is desirable to use as the ultraviolet irradiation device for treatment of a workpiece which often undergoes changes such as deformation, heat-induced color changes and the like, an ultraviolet irradiation device in which the ratio of the combined infrared light and visible rays to the total energy of irradiation of the workpiece is low and the ratio of ultraviolet light to the total energy of irradiation of the workpiece is high.
In the ultraviolet irradiation device shown in FIG. 6, the effect of reducing infrared radiation is low. Here, in the case of using the above described high pressure mercury lamp with a tube diameter of 18 mm and input power per unit length of 240 W/cm, a ratio B/A is roughly 0.69, where A is the light energy with wavelengths of less than or equal to 250 nm, which is called the energy of all of the light, and B is the light energy with wavelengths from 250 nm to 400 nm, which is called the energy of the ultraviolet light (it is ideal when B/A=1).
Furthermore, in the arrangement shown in FIG. 6, in the case of not using heat refraction filter 3 (in the arrangement described in (1) above) the ratio B/A is roughly 0.47 (in the case of a lamp input power of 240 W/cm).
On the other hand, in the combination of the cold mirror with the heat refraction filter of the water cooling type, among the arrangements described above under (2), the effect of reducing the visible rays is small; also, the ratio B/A is the same as the above described value. Furthermore, it is a disadvantage that additional space for the tube line and water supply means is needed for purposes of water cooling, because a heat refraction filter of the water cooling type is used.
As was described above, in the devices described above in (1) and (2), in any case, the effect of reducing the infrared radiation of less than or equal to 3 to 4 microns is small. Here, for a workpiece which requires a UV dose (integral irradiance of the ultraviolet light) of a stipulated constant minimum amount, a measure such as cooling of the article to be treated, or similar measures, are necessary to prevent overheating.
If, for example, pictures/letters are printed and dried on a thin plastic film with ink which is dried by ultraviolet rays, folds and the like arise in the film when it is overheated. Conventionally, therefore, there was a cooling means, and irradiation with the ultraviolet rays was performed as the film was cooled, when treating a workpiece which is often subject to heat-induced changes, such as deformations, color changes and the like.
FIG. 7 is a schematic depiction of an example of the arrangement of an ultraviolet irradiation device in which the above described cooling means is located. In the figure, workpiece W is, for example, a thin plastic film or the like, ultraviolet irradiation devices 11, 11' each have a high pressure mercury lamp, a cold mirror and a heat refraction filter, and a rotary drum 12 is provided which turns axis of rotation 13 when the workpiece W advances in the direction of the arrow in the FIG. 7. Furthermore, rotary drum 12 is tubular and water is introduced into the interior of rotary drum 12 to cool it. By means of the above described arrangement, overheating of the workpiece W by the light emitted from ultraviolet irradiation devices 11, 11' can be prevented.
As was described above, in the conventional ultraviolet irradiation device, it was considered disadvantageous to reduce the infrared radiation of less than or equal to 3 to 4 microns is a small amount. It is, then, necessary to arrange a cooling means and to treat the workpiece as it is cooled, when treating a workpiece which is often subject to heat-induced changes, such as deformations, color changes, and the like.
In the case in which, for example, the cooling means shown in FIG. 7 is provided as the above described cooling means, there is a rotary drum 12 of the water cooling type with a complex arrangement. Here, there must be a water supply pipe and a drain pipe for above described rotary drum 12; this increases system costs and operating costs.